A "twisting ball" medium for displaying an image comprising internal bichromal balls that rotate to show either black or white hemispheres in response to an externally applied electrical field, and that are contained in individual liquid filled spherical shells that are in turn dispersed in a transparent binder, which is usually solid when used.
U.S. Pat. Nos. 4,126,854 and 4,143,103 describe a twisting ball display and are incorporated by reference herein. These patents describe a display system in which the display panel is comprised of spherical balls which have an optical and an electrical anisotropy due to each hemisphere surface having a different color and electrical charge in contact with a liquid. These spherical particles are imbedded in a solid substrate and a slight space between each ball and the substrate is filled with a liquid so that the balls are free to rotate, in a changing electrical field, but can not migrate from one location to another. If one hemisphere is black and the other is white, each pixel can be turned on and off by the electrical field applied to that location. Each pixel can be individually addressed, and a full page image can thus be generated.
Most commonly the solid substrate used in this display is a gel, typically a silicone gel. The purpose of using this material lies in the remarkably large expansion in volume exhibited by gels when soaked in certain liquids, which we refer to as plasticizing liquids. 30% expansions are not uncommon when soaked in silicone oils. The bichromal balls do not expand when contacted by the plasticizing oil, so a spherical cavity opens up around each ball when the gel is immersed in a plasticizing liquid. This space fills with the plasticizing liquid.
After being soaked in plasticizer, the gel is soft and lacks durability, and must be bonded between glass or plastic sheets for protection. This process has the disadvantages that a large number of steps are required for the fabrication, adding to the cost, and that the final product lacks some of the optical and tactile properties of paper, which it emulates in some embodiments. It will be seen that significant advantages will further accrue when the limitations to planar geometries are removed. A product that does not require the use of bonding sheets would be desireable.
Historically there have been several approaches to the problem of isolating twisting balls for displays. Lee (L. L. Lee, "Matrix-Addressed Magnetic Particles Display", IEEE Trans. on Elect. Devices, Vol. ED-22, No. 9, September 1975) created a honeycomb type of structure in which the balls were contained in separate compartments which were an integral part of the substrate. Such a structure was inherently expensive to make and effectively limited the ball sizes that could be used. Later, Saitoh et al (M. Saitoh, T. Mori, R. Ishikawa, and H. Tamura, "A Newly Developed Electrical Twisting Ball Display", Proc. of the SID, Vol. 23/4, 1982) developed an isolation technique in which the individual balls were coated with a resin and subsequently dispersed in a poly vinyl alcohol substrate. After the PVA was hardened it was immersed in a solvent that had minimal effect on the PVA but that dissolved the resin around the balls, leaving them free to rotate. Once again, this is an expensive proceedure that would be difficult to implement on large scale. The most successful approach to date was that of Sheridon, described in U.S. Pat. No. 4,143,103 referenced above and described in the last two paragraphs.